Method for making the enclosure walls of a low temperature self-cleaning cooking device

ABSTRACT

THIS INVENTION RELATES TO SELF-CLEANING COOKING DEVICES WHEREIN THE CLEANING ACTION RESULTS FROM LOW TEMPERATURE, NON-CATALYTIC PYROLYSIS OF THE STAINING SPATTER AND CONDENSED VAPORS RESULTING FROM COOKING. MORE PARTICULARLY, ONE OR MORE OF THE METAL WALL MEMBERS DEFINING THE COOKING ENCLOSURE ARE FORMED WITH A GLAZED PORCELAIN ENAMEL UNDERCOATING ON WHICH AN UNGLAZED PORCELAIN ENAMEL-FORMING COATING IS PROVIDED BY UNDERFIRING A PORCELAIN ENAMEL-FORMING SLURRY CONTAINING AN EXTRA LOADING OF PULVERIZED GRANULAR MATERIALS HAVING SHARPLY ANGULAR SURFACES SELECTED FROM THE GROUP OF NON-CATALYTIC REFRACTORY OR ABRASIVE MATERIALS COMPRISING ALUMINIA, SILICA, EMERY, TUNGSTEN CARBIDE, SILICON CARBIDE, KAOLIN, AND FELDSPAR IN AN AMOUNT OF FROM ABOUT 20-30% BY WEIGHT OF THE FRIT MATERIALS IN THE SLURRY.

OCt- 24, 1972 v. A. WILLIAMms 3,700,484

v METHOD FOR MAKING THE ENCLOSURE WALLS 0F A LOW TEMPERATURE SELF-CLEANING COOKING DEVICE Filed March 2s, 1970 INVENTOR.

ATTORNEY United States Patent 3,700,484 METHOD FOR MAKING THE ENCLOSURE WALLS OF A LOW TEMPERATURE SELF-CLEANING COOKING DEVICE Victor A. Williamitis, Dayton, Ohio, assignor to [General Motors Corporation, Detroit, Mich. Filed Mar. 23, 1970, Ser. No. 21,883

Int. Cl. B44d 1/16 U.S. Cl. 117-70 A 3 Claims ABSTRACT oF THE DISCLOSURE This invention relates to self-cleaning cooking devices wherein the cleaning action results from low temperature, non-catalytic pyrolysis of the staining spatter and condensed vapors resulting from cooking. More particularly, one or more of the metal wall members defining the cooking enclosure are formed with a glazed porcelain enamel undercoating on which an unglazed porcelain enamel-forming coating is provided by underring. a porcelain enamel-forming slurry containing an extra loading of pulverized granular materials having sharply angular surfaces selected from the group of non-catalytic refractory or abrasive materials comprising alumina, silica, emery, tungsten carbide, silicon carbide, kaolin, and feldspar in an amount of from about 20-30% by weight of the frit materials in the slurry.

Recent developments in the manufacture of cooking devices have brought forth two separate techniques to aid in the cleaning of the surfaces of the cooking enclosure which are exposed to staining spatter and condensed vapors resulting from cooking. The rst system utilizes extremely high temperature pyrolysis to burn out the food residues formed on the exposed surfaces. This system has its drawbacks in that special safety interlocks and heat insulating means must be provided thus adding subtantially to the cost of the cooking device. Also, a large amount of heat must be dissipated to the environment since cleaning takes place at temperatures in excess of 900 F.

As an alternate to such system for high temperature cleaning there have been proposed low temperature catalytic oxidation systems involving the coating of the walls of the cooking enclosure with an oxidizing catalyst. Such systems are intended to effect cleaning when the air in the enclosure is heated to a temperature of about 400- 500 F. One such system is disclosed in Pat. 3,266,477 issued in the name of E. B. Stiles. The catalytic system also appears to have its drawbacks in that the catalytic materials are relatively expensive and tend to lose eiliciency over extended periods of operation, this being probably due to the fact that the internal micropore structure which represents over 90% of the catalyzing area becomes clogged so as to prevent free diffusion of the hydrocarbon materials to be catalyzed. Additionally, the coating of oxidation catalyst is extremely limited in color range and thus does not have the tiexibility from the standpoint of decorative and appealing color elect that ice is available with the use of the various porcelain enamelforming coatings known in the art. a

Of interest from the standpoint of the state-of-the-art relating to porcelain enamel nishes, the patent to Holcomb 3,338,732 discloses .a simulated aggregate finish whereby a less polished or mirror-like glossy porcelain enamel surface is obtained than would result from the normal enameling techniques. The desired surface results from the use of successive porcelain enamel-forming coatings, each coat being lired to the point of fusing or glazing the coating to develop the high gloss porcelain enamel finish, a coating of silica or like material being embedded between two successive coatings. The resultant simulated aggregate finish is Stated to produce a pleasant appearing but roughened porcelain enamel finish. Similarly, the patent to Bryant 2,466,682 discloses a porcelain enamel type member having a matte surface finish for use either as decorative structural members or as a non-slip surface in such devices as bathtubs. The patented coating in this case is achieved by the application of a porcelain enamel-forming coating followed, prior to iring, by a matte surface forming coating, the resulting composite coating being then fired. The patentee is careful to point out that the use of the matte forming materials on the enamel-forming coating during the ring operation has some physical action on the surface of the enamel to prevent the enamel from becoming glassy smooth.

In contradistinction to the above, I have developed a 10W temperature, self-cleaning system for cooking devices which provides cooking enclosure Walls having a very large surface area on which to spread the hydrocarbons spattered thereon thus enabling more rapid breakdown during the low temperature heat cleaning operation. This is achieved by the application to a glazed porcelain enamel coating of a second coating of porcelain enamel-forming slurry to which has `been added a relatively high loading of non-catalytic sharply angular granular material, this coating being underred in order to preclude glazing and the formation of a porcelain enamel linish while at the same time softening the porcelain enamel undercoat to which it then adheres.

The system by which I am able to achieve low ternperature, non-catalytic cleaning is more fully described in the specication here-following as supplemented by the drawing in which FIG. 1 shows a conventional household oven in which the walls of the cooking enclosure are coated in .accordance with my invention; and FIG. 2 is a broken away and enlarged perspective view of one of the wall members of the enclosure.

In FIG. 1 there is shown a typical household oven 1 which includes a cooking enclosure 3 having a heating element 4 and smooth-surfaced rack guide members 5 which may be removably attached to the oven walls and adapted to slidably support oven racks and utensils containing food for cooking, the temperature of the enclosure being as high as about 650 F. The spatter and overflow as well as the condensed vapors from cooking will collect on the walls of the enclosure and produce unsightly and dirtying stains which are unacceptable to the user. As noted above, high temperature pyrolysis of such staining materials has been used in cooking devices currently on the market. Also as noted above, catalyst coatings for the oven walls have been proposed for cleaning at lower temperatures in the order of 400 to 500 F.

I have found in the course of extensive testing that acceptable cleaning of the surfaces of the cooking enclosure can be obtained at temperatures of from about S50-650 F. by providing an unglazed coating having a high surface area with many jagged points on the high gloss porcelain enamel coating normally provided on such enclosure walls. Such an unglazed coating is shown in FIG. 1 as reference numeral 7.

An enlarged cross section of a portion of a coated wall surface of my invention is shown in perspective in FIG. 2 in which the metal sheet constituting the enclosure wall member is shown at reference numeral 9 and is generally formed `of sheet `iron stamped to the desired shape. In accordance with the process of my invention as hereinafter more particularly described, I apply a first coating of glazed porcelain enamel 11 and adhere on the surface of such coating an unglazed second coating 13. The second coating is underred at such a temperature as to prevent vitrification of the glass-forming materials in the porcelain enamel-forming slurry which is applied to the coating 11. While such temperature is insufficient to cause the glazing of the material to form a porcelain enamel coating, it is nonetheless high enough to soften the coating 11 in order that the materials in the underfired slurry adhere to the surface of the enamel. The normal porcelain enamel-forming firing temperature for the materials I have used is about 1410a F. and I have found that a firing temperature of about l200 F. has been sufficient to both sinter the second coating without setting-up the porcelain enamel while at the same time softening the porcelain enamel undercoat.

It should be noted that the porcelain enamel forming slurry may be any of the conventional porcelain enamel materials and compositions available on the commercial market. The porcelain enamel composition selected will of course be that which produces the desired color, White, gray or otherwise, and one which when fired to vitrification will maintain its color and glaze Without softening when operated at the temperature levels intended. It is believed unnecessary to give the formula for any of the porcelain enamels which may be used since there are a large number of publications and prior art patents from which a wide variety of compositions may be selected.

It is necessary that the second porcelain enamel coating 13, which in accordance with my method is underfired, is overloaded with pulverized granular materials having sharply angular surfaces to present a multitude of jagged points and to provide a greatly increased surface area for exposure of stain to the elevated temperature within the cooking enclosure. As noted, the granular materials are selected from the group of non-catalytic or refractory abrasive materials comprising alumina, silica, emery, tungsten carbide, silicon carbide, kaolin, feldspar and the like. I have found that about -30% by weight of the frit materials in the porcelain enamel-forming slurry used to form the unglazed coating produces the desired result while enabling processing by the techniques normally used in obtaining a porcelain enamel coating. I particularly prefer the use of alumina since it lends itself to forming a white or gray coating which is considered more pleasing to the user than the darker shades. Also, a particular form of alumina, tabular corundum, is commercially available and is characterized by sharply angular grains which particularly lend themselves to use in the coating of my invention. I have found that the grain size of the additional materials used in the second coating slurry is preferably such as will pass through a 100 mesh screen.

The unglazed wall members 7 of my invention are formed by rst cleaning the metal sheet 9 followed by the application of the porcelain enamel-forming slurry to the cleaned surface in any suitable manner commonly used in the art. By Way of example, the sheet steel may be degreased and pickled followed by the necessary washing to remove treating materials. The resulting cleaned steel surface may be coated by any suitable method such as brushing, spraying, or dipping to obtain a coating of the desired thickness. The member 9 thus coated with the first porcelain enamel slurry coat is then fired at the necessary elevated temperature to vitrify the materials and set up a glazed porcelain enamel coating.

As previously noted, not only does this base porcelain enamel coating serve as the means for anchoring the subsequent unglazed porcelain enamel-forming coating, but, it also serves as a barrier between the base sheet metal member 9 and the moisture in the cooking enclosure 3. In the absence of such a barrier, the moisture would tend to penetrate through the unglazed coating and rust the base iron sheet with resultant spalling-away of the coating.

The porcelain enamel sheet 9 is then given a second coating of the enamel-forming slurry which has been loaded with pulverized granular materials having sharply angular and pointed surfaces as distinguished from spheroidal or curved surfaced materials. As noted above, such material are selected from the group of non-catalytic or refractory abrasive materials comprising alumina, silica, emery, tungsten carbide, silicon carbide, kaolin, feldspar, and the like. Also as noted, 20-30% by weight of the frit materials in the porcelain enamel-forming coating is suitable, about 25% is a preferred amount. The thus coated sheet is then subjected to a second firing at a temperature lower than that of the first ring in order to merely soften the enamel coating while at the same time being insuciently high to develop a vitrified or porcelain enamel in the second coating. The unglazed coating is retained by the softened porcelain enamel and pr'esents a very large surface area to the cooking enclosure while at the same time being rough in texture.

My tests have indicated that coatings of the type herein described enable satisfactory self-cleaning of stained oven enclosures when heated to the elevated temperatures of from about 550 F. to about 650 F. It should of course be realized that heavier stains, as for example, coatings resulting from spill-over in cooking, will require a longer period of time for cleaning satisfactorily than will stains resulting from spatter and the like. The scope of my invention is as defined by the foregoing description and as set forth in the claims which follow.

-I claim:

1. A method of forming an unglazed coating on the metal walls of a cooking enclosure to enable self-cleaning at temperatures of from about S50-650 F. comprising the steps of applying a first porcelain enamel-forming slurry to coat the surface of a metal sheet, firing the coating to a temperature sufficiently high to form the glazed porcelain enamel, applying a second coating of the porcelain enamel-forming slurry on the glazed porcelain enamel surface, and firing said second coating to a temperature less than that necessary to form a glazed porcelain enamel and sufficiently high to merely soften the glazed porcelain enamel undercoating to retain said unglazed coating with a rough texture and a large surface area, said second slurry containing an extra loading of pulverized granular materials having sharply angular surfaces and selected from the group of non-catalytic refractory or abrasive materials comprising alumina, silica, emery, tungsten carbide, silicon carbide, kaolin, and feldspar in an amount of from about 20-30% by weight of the frit materials in said second slurry and being of a size as to pass through a mesh screen.

2. A method as set forth in claim 1 wherein said granular material is alumina and is present in an amount of about 25% by weight.

3. A method as set forth in claim 1 wherein said first slurry coating is fired at about 14l0 F. and said second v slurry coating is fired at about 1200" F.

(References on following page) References Cited UNITED STATES PATENTS Treptow 106-48 C 'Kosiorek 106-48 C Lee 106-48 C Moreland 117-70 A Adlassnig 117-70 C Lee 117-129 X Bryant 117-53 Sanford et al 117-70 C Stiles 126-19 Michael 117-129 X FOREIGN6 PATENTS 7/ 1970 Great Britain 106-48 C OTHER REFERENCES Andrews, A. I.: Porcelain Enamels, Ill., Garrard Press, 1961, p. 422.

ALFRED L. LEAVITT, Primary Examiner C. K. WEIFFENBACH, Assistant Examiner U.S. Cl. X.R.

10G-48; 117-70 C, 129; 126-19 R; 264-30, 60 

